1. Field of the Invention
This invention relates to improved silver catalysts useful for the production of alkylene oxide, their preparation, and their use in alkylene oxide processes. More particularly, the invention is concerned with preparing a supported, metal promoted silver catalyst capable of oxidizing an alkene, preferably ethylene, with an oxygen containing gas in the vapor phase to produce alkylene oxide, preferably ethylene oxide, at high efficiencies and selectivities.
2. Description of the Related Art
It is known in the art to produce supported silver catalysts for the conversion of ethylene and oxygen to ethylene oxide. Many modifications have been proposed to improve the activity and selectivity of these catalysts. These modifications have involved improvements to the supports employed, the methods of production, the physical form of the silver on the support and the inclusion of additives to the catalyst composition. Methods are known for the preparation of supported silver catalysts useful for the vapor phase oxidation of ethylene to ethylene oxide, which involve impregnating a support such as alumina with a silver salt/amine solution. U.S. Pat. No. 3,702,359 is illustrative of such procedures.
U.S. Pat. No. 2,125,333 discloses the use of alkali metals, including both sodium or potassium and their salts as additives for various silver ethylene oxide catalysts. U.S. Pat. No. 2,615,900, cites a large number of useful promoters. U.S. Pat. No. 2,773,844, discloses a multistep silver deposition process. U.S. Pat. No. 3,575,888 discloses the use of aluminum oxide supports having a pore volume between about 0.15 and 0.30 m2/gm and surface area below about 10 m2/gm. The use of small amounts of the alkali metals, K, Rb and Cs, were noted as useful promoters in supported silver catalysts in U.S. Pat. Nos. 3,962,136 and 4,010,115. U.S. Pat. No. 4,005,049 teaches the preparation of a silver/transition metal catalyst useful in oxidation reactions. In U.S. Pat. No. 4,536,482, catalytically active metals such as Ag and Re are co-sputtered along with a co-sputtered support material on a particular support. The preparation of silver catalysts which also contain alkali metal promoters by analogous procedures is shown, for example, in U.S. Pat. No. 3,962,136. Similar procedures for the preparation of silver catalysts promoted by an alkali metal and rhenium and also with a co-promoter selected from sulfur, molybdenum, tungsten, chromium and mixtures are shown in U.S. Pat. No. 4,766,105. Catalyst preparation by these prior art procedures has involved impregnating a support with a silver/amine solution which may contain the various promoters, and thereafter heating the impregnated support in a forced air oven up to a temperature of about 275° C. in order to reduce the silver to metallic silver and to separate volatiles from the catalyst.
U.S. Pat. No. 4,916,243 show silver catalysts for ethylene oxidation to ethylene oxide prepared by impregnating an inert support with a silver/amine and silver lactate solutions. The impregnated carriers were than heat treated on steel belt transported through a 2″×2″ square heating zone for 2.5 minutes, the heating zone being maintained at 500° C. by passing hot air upward through the belt, or at 400° C. for 4 minutes.
U.S. Pat. No. 5,444,034 relates to silver catalyst preparation wherein a support is impregnated with a hydrocarbon solution of a silver salt of an organic acid and activated in stages up to a temperature of 500° C. under an inert gas such as nitrogen.
In other descriptions of processes of ethylene oxide production addition of oxygen-containing gases to the feed increased the efficiency. For example in U.S. Pat. No. 5,112,795 5 ppm of nitric oxide was added to the gas feed of composition: 8 volume % oxygen, 30 volume % ethylene, about 5 ppmw ethyl chloride and the balance nitrogen.
In the other processes to increase efficiency, particularly selectivity, the catalyst were treated at certain temperature and certain gas mixture. For example in U.S 2004/0049061 and 2004/002954 the selectivity of a highly selective epoxidation catalyst can be improved by heat-treating the catalyst in the presence of oxygen at a temperature which is typically above the catalyst's normal initial operation temperature.
U.S. Pat. Nos. 5,504,052 and 5,646,087 show silver catalysts for ethylene oxidation to ethylene oxide prepared by impregnating an inert support with a silver/amine solution as well as with various promoters and calcining the impregnated support at 300° C.-500° C., while the catalyst is maintained under an inert atmosphere. Thus the prior art teaches catalyst preparation by calcining an impregnated support either in air, i.e. a large amount of oxygen, or under an inert atmosphere such as nitrogen. It has been now been surprisingly found that the calcination of an impregnated support in an inert atmosphere, such as nitrogen, with the addition of only a small amount of an oxidizing gas, such as molecular oxygen in the inert atmosphere, improves the effective life, activity and selectivity of an ethylene oxide catalyst.